Metalliferous mine tailings in arid regions pose a significant health risk to proximal populations because they are prone to wind-borne dispersion and water erosion. The problems are extensive and persistent as impacted sites lack normal soil stabilization. Phytostabilization is the revegetation of mine tailings to ameliorate these issues with the goal of root zone metal accumulation to avoid metals from entering the food chain through above-ground biomass. The role of plant roots and microbes in promoting mineral dissolution-precipitation reactions and associated metal sequestration is an active area of research, but little is known about reaction trajectories and changes in particle-scale metal speciation of plant-tailings systems, owing largely to their geochemical heterogeneity and microbial complexity. Since the form or speciation of a metal controls its bioavailability and toxicity, research that probes coupling between metal speciation and microbial dynamics in response to phytostabilization is needed. The overarching goal of the proposed work is to identify multi-scale process-links between biological structure and contaminant geochemistry during phytostabilization of mine tailings. The four Specific Aims are: (i) to deduce the dependence of metal(loid) molecular environment on particle-specific weathering processes;(ii) to assess the spatial correlations between biogenic (and geogenic) weathering products and specific microbial cells and biofilms;(iii) to relate these direct observations of solid phase biogeochemistry with time- and depth-resolved measurements of tailings pore waters (focusing on the mobility and bioavailability of metal contaminants);and (iv) to measure the influence of solid and solution phase dynamics [(i) through (iii)] on the evolution of tailings microbiology and geochemistry both in the rhizosphere and in the bulk tailings over the course of phytostabilization. Embedded within these objectives is the additional goal of statistically integrating the nano- to macro-scale "geo" and "bio" information gained to better understand the phytostabilization process and possible outcomes in terms of exposure and toxicity risks associated with tailings sites in arid environments. Biostabilization will be probed over a 27 month mesocosm experiment using an array of advanced tools that can interrogate the complex associations of roots, microbes, minerals and metals at high spatial resolution. A time series of bulk and micro-focused X-ray spectroscopic, molecular biology/microbial ecology, and aqueous geochemical data will be generated for analysis of coupled processes that control the local contaminant environment. To assess how these process-links affect the larger goal of metal stabilization, we will coordinate our "bio" and "geo" observations so that they probe identical locations, together traversing molecular to macroscopic (mesocosm-level) scales. This research is both timely and necessary as growth in the US Southwest is exploding and communities are being developed in closer proximity to such tailings sites.